Ethylene is produced by feeding the naphtha and steam into a cracking tube and heating the tube from outside to a high temperature in excess of 1000.degree. C. to crack the naphtha inside the tube with the radiation heat. Accordingly, the material for the tube must be excellent in resistance to oxidation and in strength at high temperatures (especially creep rupture strength and creep deformation resistance).
The process for cracking the naphtha forms free carbon, which becomes deposited on the inner surface of the tube. If carbon is deposited which is small in thermal conductivity, the tube needs to be heated from outside to a higher temperature to cause the cracking reaction, hence a lower thermal efficiency. The tube material must therefore be highly resistant to carburization.
Improved HP material (0.45 C-25 Cr-35 Ni-Nb,W, Mo-Fe) according to ASTM standards has been in wide use as a material for cracking tubes for producing ethylene. With an increase in operating temperature in recent years, however, this material encounters the problem of becoming impaired greatly in oxidation resistance, creep rupture strength and carburization resistance if used at temperatures exceeding 1100.degree. C.
Accordingly, the present applicant has already developed a material capable of withstanding operations at high temperatures above 1100.degree. C. (Examined Japanese Patent Publication No. SHO 63-4897). This material comprises, in % by weight, 0.3-0.5% of C, up to 2% of Si, up to 2% of Mn, 30-40% of Cr, 40-50% Ni, 0.02-0.6% of Al, up to 0.08% of N, 0.3-1.8% of Nb and/or 0.5-6.0% of W, 0.02-0.5% of Ti and/or 0.02-0.5% of Zr, and the balance substantially Fe.
Although this material is usable for operations at high temperatures over 1100.degree. C. with sufficient oxidation resistance, high creep rupture strength and excellent carburization resistance, it has been found that the material undergoes creep deformation relatively rapidly at high temperatures and still remains to be improved in weldability.
If the creep deformation resistance is small at high temperatures, permitting deformation to proceed at a high rate, the guide supporting the cracking tube comes into bearing contact with the furnace floor to induce the bending of the tube. When deformed by bending, the tube is locally brought closer to the heating burner, and the local tube portion is heated to an abnormally high temperature, which results in deterioration of the material and accelerated carburization. To diminish such deformation, the secondary creep rate must be low.
With cracking tubes, it is required to remove the portion deteriorated by carburization, bulging or the like for replacement and repair by welding. Nevertheless, if the material is not satisfactorily weldable, it is substantially impossible to locally repair the tube, giving rise to a need to replace the faulty tube by a new one to entail a very great economical loss. Improved weldability can be imparted to the material by enhancing the ductility thereof after aging.
We have conducted intensive research and found that in the case of the above-mentioned alloy material, Cr incorporated therein to assure oxidation resistance and strength at high temperature is present in an excessive amount and therefore upsets the quantitative balance between Cr and Ti or Zr which is incorporated in the alloy to retard the growth and coarsening of Cr carbide formed in the austenitic phase and to thereby afford improved creep rupture strength, consequently diminishing the creep deformation resistance.
Accordingly, we decreased the Cr content to thereby optimize the quantitative balance between Cr and Ti and/or Zr, retard the progress of secondary creep and improve the ductility after aging.
We have also found that Nb-Ti carbonitride contributes a great deal to the improvement in creep rupture strength. Nitrogen is therefore made present in an increased amount to form the Nb-Ti carbonitride to ensure high creep rupture strength.